Global Navigation Satellite System (GNSS) is the commonly accepted term for positioning systems based on line of sight radio from orbiting satellites. GNSS positioning applies the simultaneous range to a minimum of four GNSS satellites from a receiver, along with the known satellite coordinates obtained from the broadcasted navigation messages, to determine the three dimensional coordinates of the receiver position and a receiver clock offset. Existing GNSSs include the Global Positioning System (GPS) funded and controlled by the U.S. Department of Defense, the GLObal NAvigation Satellite System (GLONASS) founded and controlled by Russia, the GALILEO system founded and controlled by Europe, the Beidou system founded and controlled by China and the Quasi-Zenith Satellite System (QZSS) founded and controlled by Japan.
For a typical GNSS system, the navigation satellites transmit two types of measurements; the code portion allowing a code pseudorange measurement to be determined and the carrier portion allowing a carrier phase measurement to be determined. The code pseudorange is an unambiguous measurement of distance to the satellite transmitting the signal, but with relatively poor measurement accuracy. The carrier phase measurement has better measurement accuracy, but always contains an ambiguity due to the unknown number of carrier wavelengths existing in the phase measurement.
However, since the signals being transmitted between the satellites and the receiver are subjected to numerous errors, such as satellite orbit and clock errors, atmospheric delay, environmental effects and the like, position determined even using the carrier portion is not completely accurate. Augmentation systems, which determine errors and provide corrections to code pseudorange measurements and carrier phase measurements, have been developed to mitigate these errors. The augmentation systems can be ground based or satellite based and some freely provide the corrections whereas others require a subscription in order to use the corrections. These augmentation systems include code pseudorange based augmentation systems such as the Wide Area Augmentation System (WAAS) covering North America, the European Geostationary Navigation Overlay System (EGNOS) covering Europe, the Multifunctional Transport Satellite Space bases Augmentation System (MSAS) covering East Asia, GPS Aided Geo Augmented Navigation (GAGAN) covering India, and local DGPS systems to provide code pseudorange corrections and carrier phase based systems, such as OmniStar™, StarFire™ and CORS systems to provide carrier phase measurement corrections.
A typical existing GNSS receiver can apply code corrections or phase corrections from publicly available systems or privately owned systems. Code corrections generally offer less of an accuracy improvement as compared to phase corrections. However, phase correction based systems have a relatively longer initialization period, typically 20 to (30 minutes. There is a need in the art for methods and systems to optimally combine these two kinds of corrections in one receiver to achieve a positioning result that has the fast initialization of code corrections and the higher accuracy of phase corrections.